In the field of projectors, optical lens systems and similar applications requiring a relatively intense source of light, it has been a common practice to employ a light source in the form of a tungsten lamp. Although tungsten or tungsten halogen lamps do have certain desirable features such as low cost, desirable color features enhancing skin tones and do not require a special power source, several undesirable features are unfortunately also present. For example, structures employing a tungsten source do not generate enough blue light, tend to undesirably generate large amounts of heat which necessitates expensive and cumbersome cooling devices located adjacent the light source, and tend to exhibit a relatively short life such as an operational period of about 10 to 20 hours. Thus, it is not uncommon to replace light source each time the apparatus is employed. Obviously, such inconvenience and expense leaves much to be desired. Moreover, screen illumination is limited due to the inability to increase surface luminance much beyond 3400.degree. K. while the mechanical body structure is rigid leading to destruction during operation by chemical means and by vibration or shock.
An improvement over the above-described system is provided by the use of metal halide discharge lamps as a light source. For example, a common form of high pressure metal halide discharge lamp is disclosed in U.S. Pat. No. 4,161,672. Therein, a double-ended arc tube or an arc tube having electrodes sealed into diametrically opposite ends is employed in conjunction with an evacuated or noble gas filled outer envelope. However, it has been found that such structures are relatively expensive to manufacture and are obviously not appropriate for use in projectors or other optic lens-type appartus.
As to single-ended metal halide arc discharge lamps, U.S. Pat. Nos. 4,302,699; 4,308,483; 4,320,322; 4,321,501 and 4,321,504 all disclose variations in structure or fill suitable to a particular application. However, any one or all of the above-mentioned patents leave something to be desired insofar as stability of the arc and isothermal uniformity of the discharge lamp are concerned.